The Cyclors, also known as the Cylors (COMIC: The Then and the Now) were a race who originated from a higher dimension. (COMIC: Pull to Open) The War Doctor considered them "dimensional nomads" who were "functionally omnipotent".
The Cyclors arrived on the Overcast's plane of reality because of the Overcast's collective subconscious which created a psychic tether between them. They fed on the bond they had with the Overcast like parasites. (COMIC: The Organ Grinder) The Cyclors raised the Overcast from savages to being a peaceful race. For twelve hundred years the Overcast prospered due to their help. (COMIC: The Then and the Now)
During the Last Great Time War, the Cyclors annihilated the spiral arm of a galaxy. The Daleks were the first to recruit them, as they offered them a sensation they had never experienced: genocide. (COMIC: The Organ Grinder) The War Doctor was sent to deal with them (COMIC: Pull to Open), accompanied by the Master. (COMIC: The Organ Grinder)
The Cyclors were warped by Alice Obiefune's activation of the Psilent songbox. The energy of the Songbox also touched a Volatix Cabal Dalek mutant, who deformed them further, turning them into the Malignant. (COMIC: Fast Asleep) The Malignant wiped out over fifty generations of the Overcast. (COMIC: The Then and the Now)